1. Field of the Invention
The present invention concerns reducing electromagnetic radiation generated by high speed digital signals in computer systems by using grounded conductive traces circumscribing the internal layers or planes of printed circuit boards.
2. Description of the Related Art
Computer systems are being designed to operate at lower power and higher speeds. For example, typical personal computer systems operate at clock speeds of 25 to 66 megahertz (MHz) or more and are designed to operate more efficiently and consume less power, with frequency and efficiency increasing almost daily. In general, the higher the frequency of the clock and other digital signals of the computer system, the more operations the computer system can perform per second. Higher frequency signals, therefore, are deemed desirable since they allow faster operation which enhances system performance. Most digital computer systems use digital logic signals based on square waves, so that higher frequency digital signals include faster rising and falling edges. The fast rise and fall edges include very high frequency content which increases the electromagnetic energy of the digital signals. The increased electromagnetic energy, in turn, increases the electromagnetic radiation (EMR) produced by the digital signals, which may then be received and cause electromagnetic interference (EMI) in nearby circuitry and electronic devices, such as aircraft navigation, commercial radio systems, televisions and other communications equipment.
The EMR generated by high speed digital signals is typically coupled through stray capacitance of nearby circuitry, resulting in EMI in the form of current or voltage spikes. The problem of EMI is exacerbated by the fact that electronic devices are being designed at lower power levels, implying higher impedance circuitry, where the higher impedance tends to amplify the electronic noise. In computer systems, the faster edges and higher frequencies are used to effect higher resolution, so that much less output noise is permitted. Even relatively small amounts of stray capacitance can couple significant noise levels if exposed to EMR caused by high speed logic signals. Reducing stray capacitance is desirable, but it is very difficult, if not impossible, to remove all stray capacitance. In any event, the primary focus for most electronic design engineers is on reducing EMR. Since high speed digital signals are desirable to improve system performance in spite of the additional electronic noise levels, isolation of the source and reduction of EMR is the goal, especially in computer systems.
The U.S. Federal Communications Commission (FCC) regulates the amount of radiated and conducted emissions generated by electronic devices including computer systems and peripheral devices, and has developed a classification system indicating acceptable levels of EMR for particular uses. It is very desirable to keep the standard high, and thus the EMR low, when designing electronic devices for home use, since the typical home includes a high number of susceptible electronic devices, such as televisions, video cassette recorders, stereos and many other home electronic devices. The FCC must certify each piece of hardware including computer systems and peripheral devices before the hardware can be sold of even displayed at a trade show. The FCC defines two levels of certification of electronic devices based on the amount of non-ionizing radiation emitted at certain specified distances. A class A device is suitable for commercial use only, where EMR is measured at 10 meters and must not exceed 90 microvolts per meter (.mu.V/m) for the 30-88 MHz frequency range, 150 .mu.V/m for the 88-216 MHz range, 210 .mu.V/m for the 216-960 MHz range and 300 .mu.V/m above 960 MHz. A class B rating is met when the EMR measurements taken at 3 meters do not exceed 100 .mu.V/m for 30-88 MHz, 150 .mu.V/m for 88-216 MHz, 200 .mu.V/m for 216-960 MHz and 500 .mu. V/m for frequencies greater than 960 MHz. A class B rating is a tighter standard desirable for home use and for portable computer use. Computer designers must constantly seek new and improved methods of reducing EMR in order to qualify for a class B rating as system frequencies increase. Due to the ever increasing demand for greater capabilities at lower power, however, it is becoming more difficult to meet this standard. In fact, the first prototype versions of new computer systems rarely qualify for class B, so that engineers spend significant amounts of time identifying sources of EMR and designing fixes to reduce EMR.
The high frequency digital signals necessary to perform the function of a computer system are transmitted from one component to the next on conductive traces routed on planes or layers of multilayer printed circuit boards (PCBs). These signal traces are a primary source of EMR. EMR is also conducted to the power plane of the PCB through common impedance noise coupling. Electrostatic shields can be very effective in isolating, reducing or even eliminating EMR. One of the most effective electrostatic shields is a metal box or a closed metal shell comprising a low impedance element preferably coupled to ground or to a common voltage level. The underlying principle is that a charge from an external potential can not exist on the internal portion of the shield, and vice versa, so that the electric field is theoretically eliminated. It is impracticable to implement completely closed metal shells in computer systems, so that shields are typically implemented as partial shells, or metal walls and barriers. Even partial metal shields, however, increase cost, add weight restrict airflow and consume Valuable space. Furthermore, metal shields are difficult and inconvenient to implement for use with PCBs.
Therefore, it is desirable to provide an apparatus or method to significantly reduce EMR generated by high speed digital signals residing on internal planes of PCBs without adding components, consuming space, increasing weight or significantly increasing cost.